In conventional AFM lithography, a pattern formation is mostly performed by preparing self-assembled monolayers using an organic resist [Jpn. J. Appl. Phys., 37, 7148, 1998, Kim J. C., J. Kor. Phys. Soc., 35, 1013, 1999, Kim, J. C., Adv. Mater., 12, 6, 424, 2000, Rivka M.].
The process of oxide pattern formation by AFM is as follows. An organic thin film is formed on a silicon substrate with a predetermined thickness. Then, a voltage in the order of several volts is applied locally using an AFM tip to form an oxide pattern. The resultant silicon oxide is etched much faster than other parts because it has a loose structure. The part of the organic thin film to which the voltage has not been applied can be used as a resist during etching to obtain a wanted positive pattern. However, in practice, it is difficult to completely remove the thin film after pattern formation because of its strong chemical bonding with the substrate.
In AFM lithography, applied voltage, electric current, scanning rate, humidity, capacity of resist, etc., are important factors [Vac. Sci. Technol., 1223, 1996, Sugimura, A., J. Vac. Sci. Technol., 2912, 1997, Birkelund K., J. Appl. Phys. Lett., 285, 1997, Avouris P.].
Unless lithography is performed under optimum conditions, patterns with irregular linewidth and broken lines are obtained. For better pattern formation, the development of high performance resist is required and such conditions as applied voltage, scanning rate and humidity should be appropriately controlled.
Sulfonium is usually used as a photoacid initiator or radical photoinitiator in polymerization or as an acid catalyst generator to deprotect organic compounds. A variety of uses have been developed for sulfonium, which generates cation photoacid initiators when excited by UV in the specific region. And, with the recent development in electronics, it is used for micropattern formation in microelectronics. The sulfonium cation photoinitiator has good photopolymerization efficiency as it produces strong acid when exposed to light, but is disadvantageous in that it is less soluble in organic solvents.
With regard to this problem, there is a report that the use of non-ionic photoinitiators generating acids by exposure to light improves solubility in organic solvents [Journal of Photopolymer Science and Technology, Vol. 13, No. 2(2000), pp 223-230]. However, the acids generated by the non-ionic photoinitiators are weak acids such as methylsulfonic acid, propylsulfonic acid and camphosulfonic acid, not the strong trifluoromethanesulfonic acid.